The fluids produced from subterranean reservoirs are typically a mixture of oil, water, gas, normally with a quantity of sand. In the early stages of production the fluids may be “dry”, i.e., containing little or no water, but as production continues it is normal for greater quantities of water to be produced. In mature fields the proportion of water in the produced fluids can reach 90% and possibly around 95%. Reservoirs do not always produce large quantities of sand but when they do the sand production generally increases as the water production increases. The water produced from the reservoir is termed “produced water”.
The produced water may be injected back into the formation or it may be discharged into the environment; for example, in off-shore production areas the water may be discharged into the sea. However, the produced water must be thoroughly treated in order to remove substantially all traces of oil and gas prior to being discharged into the environment in order to meet extremely strict environmental regulations.
In conventional production operations the produced fluid from the reservoir is initially treated to separate the saleable oil and gas from the un-saleable water and sand. However, the separated water will typically still contain unacceptable quantities of oil and gas and as such is normally subject to a secondary treatment to further reduce the concentration of hydrocarbons to acceptable levels for discharge to the environment or injection back into the formation.
A process known as “flotation” in commonly used to assist in the removal of oil and other contaminants from water. The principle of flotation is that bubbles of gas are introduced into (e.g., Induced Gas Floatation) or are established in (e.g., Dissolved Gas Floatation) a vessel containing a contaminated water, in which the bubbles will to a greater or lesser degree attach to the contaminants, such as oil droplets, and drag them to the surface of the water, leaving the bulk of the water depleted of contaminants, and the upper layers of the water enriched with contaminants. In subsequent discussions, each volume of water to which gas bubbles are added or are created to separate contaminants may be referred to as a “cell” or “flotation cell”.
Flotation is usually operated as a continuous process, where there is a continuous inflow of contaminated water into the cell and a continual outflow of contaminant enriched water drawn from the surface layers of the cell and a continual outflow of the contaminant depleted water from the cell at a rate so as to maintain an essentially constant level in the vessel.
It is usual for the contaminants floated to the surface of the water to be retained in a froth which is either formed naturally when the contaminants are present at the higher concentrations found at the water surface, or with the assistance of chemicals which are added to the inflowing liquid. Buoyant contaminants, for example droplets of oil, may not need to be frothed to keep them at the surface.
The contaminants on the water surface may be removed by a variety of means, the two most common being weirs set slightly below the water surface so that the contaminant enriched surface layer preferentially flows over them, or paddles which sweep the contaminant enriched surface layer over a weir which is normally set slightly above the water surface. A number of designs of floating skimming devices are also known which have the advantage that they can tolerate a wider variation in operating liquid level than either of the aforementioned fixed weir methods can accommodate.
In Induced Gas Floatation (IGF) methods, gas bubbles are typically added to the contaminated water by eductors or mechanical mixers. An IGF cell must be mixed to bring the gas bubbles into intimate contact with the contaminants, such as oil droplets, so that they can be separated by them, but this mixing has the side effects of making it more difficult for the bubbles to rise to the surface, and causing variation in the residence time of parcels of water within the cell. While the average residence time in the cell can be determined by the volume of the cell and the water flowrate, the mixing can mean that some parcels of water pass through the cell in a much shorter time than is required for good separation, and conversely some parcels of water may reside in the cell much longer than the average residence time.
The necessity to have mixing in the cell to contact the gas bubbles and contaminants reduces the efficiency of separation in the cell. For this reason IGF vessels are commonly horizontal vessels which contain a plurality of IGF cells in series, typically four, so that the overall efficiency of the separation is increased. Examples of known IGF systems of this type are shown in U.S. Pat. No. 4,564,457, US 2006/0213840, U.S. Pat. Nos. 3,972,815, 3,647,069 and 5,348,648.
Nevertheless, in some applications vertical single stage cell flotation units are known, which have a single IGF cell which typically has a cell volume, and hence residence time, somewhat larger than would be found in the four cells of a tyicall horizontal IGF unit. Examples of such IGF arrangements are shown in WO 2004/112936 and U.S. Pat. No. 5,011,597.
It is understood that an IGF unit requires the fluid to reside within the vessel for a period sufficient to allow floatation of the oil and separation of the gas. However, increasing residence time will directly reduce the maximum fluid treatment rate which can be achieved. It may be possible to address this issue by increasing the size of the IGF unit but this is not desired due to the restricted available space in conventional production environments.
It is desirable to reduce the space occupied by, and weight of, equipment installed on offshore platforms, and for this reason compact floatation units have been proposed in the art for treating produced water with minimal plant footprint. For example, prior art reference WO 02/41965 discloses a vertically arranged vessel which receives fluid to be treated via a tangentially arranged fluid inlet. Arranging the fluid inlet in this manner establishes rotation of the fluid within the vessel which is alleged to assist coalescence of oil and gas bubbles and floatation to the surface. The vessel may incorporate a spiralling guide vane to enhance fluid rotation.
In WO 02/41965 the vessel is operated at a low pressure to permit dissolved gas to evolve from the water phase and create gas bubbles in the zone adjacent the fluid inlet to mimic the effect of IGF units. However, if an insufficient volume of gas is present in the fluid then additional gas may be added to the fluid.
Prior art reference EP 1 400 492 also discloses a compact floatation unit which comprises a vertically arranged vessel with one or more tangential fluid inlets to encourage fluid rotation. In EP 1 400 492 the vessel also includes tangentially arranged fluid/sparge gas inlets. These inlets communicate gassified water into the vessel.
Other techniques for treating fluids by the use of gas bubbles include cascade floatation techniques, in which fluid to be treated is passed through eductors into containers, in a cascading fashion. Examples of such techniques are disclosed in U.S. Pat. Nos. 1,311,919, 1,380,665 and 4,406,782.
It is among objects of the present invention to obviate or mitigate one or more problems in the prior art.